Thermostatic switch



Nov. 27, 1962 H. D. EPsTElN 3,066,205

THERMOSTATIC SWITCH Filed March 1e, 195s United States Patent 'O Thisinvention relates to thermostatic switches, and more particularly toswitches for motor protection circuits and the like.

Among the several objects of the invention may be .noted the provisionof a thermostatic switch for motor protection circuits adapted tominimize overswing of temp perature caused =by high currents such asflow under locked rotor conditions, while at the same time providing formore positive make and break contact action; and the provision of aswitch ofy this class which is simple and compact in form, adaptedreadily to be located in or on circuits such as motor windings to beprotected. Other objects and features will be in part apparent and inpart pointed out hereinafter. l Theinvention accordingly comprises theelements and combinations of elements, features of construction, andarrangements of parts which will be exemplified in the structureshereinafter described, and the scope of which will be indicated in thefollowing claims.

In the accompanying drawings, in which several of various possibleembodiments of the invention are illustrated,

FIG. 1 is an axial section showing one form of the invention;

FIG. 2 is a right-hand end view of the switch shown Y inFlG. l;

FIG. 3 is a cross section taken on line 3 3 of FIG. 1;

FIG. 4 is an end view showing another form of the invention; l Y

FIG. 5 is an axial section taken on line 5--5 of FIG. 4; and,

FIG. 6 is an axial section showing a third form of the invention.

Corresponding Ireference characters indicate corresponding partsthroughout the several views of the drawings.

Referring now more particularly to FIG. l, there is shown at numeral 1an elongate tubular switch casing or support composed, for example ofmetal, although it may be formed of other materials such as aninsulating material, if desired. At numeral 3 is shown a firstrelatively long bimetallic thermostatic strip or blade having bondedcomponents 5 and 7. Component 5 has a lower coeflicient of thermalexpansion than does component 7. One end of the strip 3 extends from thecasing 1 through an encirclement of insulating material 11 to form a terminal 9. The casing 1 is crimped around the material 11, as shown at 13.This anchors strip 3. A contact 15 is carried at the inner free end ofthe strip 3.

Numeral 17 indicates a second bimetallic strip or blade which is shorterthan the strip 3 and extends outward to form a terminal 19. Thisextension is through an encirclement of anchoring insulating material 21about which the casing is crimped as shown at 23. Strip 17 has acomponent 18 having a coefcient of thermal expansion which is lower thanthe coefficient of thermal expansion of its other component 20. At theend of the strip 17 is carried a contact 25.

The strip 17 has a greater electrical resistance and a lower thermalmass than strip 3, as for example, by 4making this strip 17 of higherresistance material and shorter or narrower or thinner than strip 3, orany suitable combination of these. The lower thermal mass of strip 17results in its relatively rapid heating and cooling, as compared withthe cooling of strip 3, in response to overload current.

3,066,205 Patented Nov. 27, '1962 It is preferred that strip 17 beshorter than .strip 3, whereby its reaction is increased againstdownward bending in response to pressure between contacts 15 and 25.This accelerates the time at which the switch -tively long length ofstrip 3 and the vrelatively confined .final movement of contact causedby the relatively reaches an opening state under given heatingconditions. Nevertheless, this does not interfere with desired delayedvswitch-closing action, as will appear.

Assume that the device is wired into a circuit supplying a motor windingto be protected under conditions including high overload current, forexample locked rotor current. The strip 17, due to its greaterresistance, will heat ,causes contact 25 to tend to overtake contact 15and temporarily increases the contact pressure between them. Thiscontinues until strip 3 rises in temperature sufh- -ciently to increaseits rate of recessive movement in the direction of dart a. Finally,contact 15 will rapidly separate from contact 25, thus opening thecircuit. This final rapid opening action is due to the large freemovement of contact 15 (when it moves) offered by the relashort lengthof strip 17. The circuit-opening action or trip is quite rapid andpositive.

After the circuit has opened, the strip 17 in the absence of currentwill cool much more rapidly than will strip 3, due to its considerablylower thermal mass occasioned by the relatively small volume of materialcontained in it,

as compared to the relatively large volume of material contained in thestrip 3. Thus, upon cooling, contact 25 is rapidly withdrawn a largedistance from contact 15, leaving a comparatively large gap to betraversed by the return movement of contact 15 before the circuit canreclose. Strip 3, having the higher thermal mass, will cool off moreslowly, and this fact, coupled with the existence of this greater gap,will provide a relatively long period before reclosure of the circuitwill occur. In other words, the retractive movement of contact 25 ismore rapid than that of Contact 15. Such a condition is advantageous, inthat the motor winding thus has increased time within which to coolbetween cycling events, thereby minimizing temperature rise in the motorwinding under cycling conditions.

In FIGS. 4 and 5 is shown another form of the invention, in which allparts that bear the same numbers as in FIG. 1 are the same. Thedifference in this case exists in the manner in which the casing 1 iscrimped at its lefthand end, the insulation at this point being omitted.The metal casing 1 in this construction is crimped directly to the strip17,'as shown at numeral 27, and may be welded thereto as shown at 29.This provides for a heat-conductive connection between them, wherebyambiently applied heat will flow more rapidly to and from strip 17 thanstrip 3. This also has the effect of making the casing 1 available as aline terminal, if desired. Thus this form of the invention respondseffectively to temperature changes caused by ambient heating, as well assuch changes caused by dow of current through the strips.

In FIG. 6 s shown another form of the invention, of reduced length. Inthis example, the metal casing which is cup shaped is shown at numeral31. yInsulation is shown at 33, surrounding the terminal end 35 of thestrip 37. The crimping of the casing around the insulation 33 andterminal 35 is shown at 39. The strip 37 has its component of highcoetticient of thermal expansion indicated at 43, and the component ofrelatively low coefficient of thermal expansion at 41. It will beunderstood that the strip 37 is comparatively long and has acomparatively large heat mass. It carries at its free end a contact V45.

At numeral 47 is shown a shorter bimetallic strip, againV having a lowthermal mass and a relatively greater resistance, designed forcomparatively rapid heating upon application of current andcomparatively rapid cooling in the absence of current. This strip 47 hasa component of relatively high coe'lcient of thermal expansion shown at51, and a component of relatively low coefficient of thermal expansionshown at 49. One end of the strip 47 is anchored to the casing 31, as bymeans of welding '3. Its other and free end carries a Contact 55. Theoperation of this form of the invention is simliar to that abovedescribed. Upon application of high overload current, strip 47 heatscomparatively rapidly and strip 37 comparatively slowly. The resultingmotion of the end of strip 37 in the direction c is therefore yovertakenby the resulting motion of the end of strip 47, shown at d. This temporarily increases the contact pressure. Then as strip 37 heats and itstemperature rises, the contact 45 withdraws from contact 55 to open thecircuit. The result is immediate and rapidl cooling of strip` 47, whichsends Contact 55 into a position providing a large gap between contacts,which is -only slowly closed upon slow recooling of the relatively highheat mass strip 37. The result is that this form of the invention hasthe advantages above referred to in connection with the forms shown inFIGS. 1-4. It also has the advantage of the FIG. 5 form in that itresponds eiectively to temperature changes induced by ambient heating,this being due to the heat-conductive connection between the metalcasing 31 and the strip 47.

In view of the above, it will be seen that the several objects of theinvention are achieved and other advantageous results attained.

As various changes could be Imade in the above constructions withoutdeparting from the scope of the invention, it is intended that allmatter contained in the above description or shown in the accompanyingdrawings, shall be interpreted as illustrative and not in a limitingsense.

I claim:

1. A thermostatic switch comprising rst and second electricallyinsulated thermostatic strips, a heat-conductive metal casingsurrounding said strips, each strip being anchored at one end thereof tosaid casing and being free at its other end, said free ends carryingengageable and disengageable iirst and second contacts, the rst strip inrespect to the second strip being comparatively shorter, ofcomparatively higher resistance and of comparatively lower heat mass,said casing being in direct heat-conducting contact with the shorterstrip at the point at which it is anchored and including electrical andheat insulation between the casing and the longer strip at the point atwhich it is anchored, each strip having components of relatively highand low coeflcients of thermal expansion arranged for contact movementsin the same general direction in response to temperature changes,whereby in response to heating of the strips, contact pressure isinitially increased by overtaking movement of the first contact relativeto the second contact, the contacts Ibeing thereafter rapidly separated,and in response to cooling, the rate yof retractive movement of theiirst contact relative to the second contact is greater, therebydelaying switch closure.

2. A thermostatic switch according to claim l, wherein saidheat-conductive contact between the shorter strip and the casing islocated adjacent an intermediate portion of said second strip and thestrips extend generally in the same direction from their respectivepoints at which they are anchored.

3. A thermostatic switch according to claim l, wherein the strips extendin opposite directions from the points at which they are anchored to thecasing to form terminals.

References Cited in the le of this patent UNITED STATES PATENTS1,743,053 Traver Jan. 7, 1930 1,916,669 Kuhn et al. July 4, 19331,916,671' Hanser et al. Iuly 4, 1933 2,320,811 Cook June 1, 19432,338,474 Wilson Ian. 4, 1944 2,586,309 IDales Feb. 19, 1952 2,627,003Porter Ian. 27, 1953 2,792,474 Dales May 14, 1957 FOREIGN PATENTS597,603 France Sept. 5, 1925

